32 BULLETIlSr 1121, U. S. DEPAETMENT OF AGRICULTURE. 



it seems difficult to believe that the superioity of the crossbreds is due 

 merely to dominance of factors present in some of the inbred families, 

 but absent in others, the hypothesis which will be at the basis of our 

 later discussion. We are, however, able to analyze the character 

 which we are considering into four components. The crossbreds do 

 not exceed the best inbred family to anything like as great an extent 

 in each of these component characters. The superiority of the cross- 

 breds is consistent, however, and hence cumulative, in comparison with 

 the superiority of the best inbred family over the average, the ''best" 

 inbred family being one family in one case and another in another. 

 The four component characters are themselves doubtless highly com- 

 plex genetically. If further analysis were possible, it might well turn 

 out that dominance of the factors tending toward vigor in each respect 



is not even perfect. 



COAT COLOR. 



Six series of Mendelian factors are known which affect color in 

 guinea pigs. Among our five leading inbred families there are 

 variations in only two of these series, the albino series (C, c^, c'^, C", c^) 

 and the agouti series (A, a', a). All five families are piebald (s) 

 instead of self (S), tortoise shell (e') instead of self -black (E) or self- 

 red (e), black (B) instead of brown (b), and black (P) instead of 

 pink-eyed pale sepia (p) . 



Table 11. — Color pattern and factorial composition of the Jive principal inbred families . 



Family. 



Color. 



Factorial 

 composition. 



Black-red- white tricolor 



/Black-red- white tricolor 



\0ccasional albinos 



Agouti-red- white tricolor , 



YeUow agouti-yellow-white tricolor. 



Agouti-red-white tricolor 



CCaa. 

 CCaa. 

 ccaaa. 

 CCAA. 



CkCkAA. 



CCAA. 



First crosses between Families 2 and 13 produce black-red-white 

 tricolors. All other first crosses among these five families produce 

 agouti-red-white tricolors, except that occasionally cream agouti- 

 cream-white tricolors (c'^c^Aa) appear in crosses between Families 

 35 and 13. Segregation takes place in regular Mendelian fashion in 

 later generations, there being no linkage between factors C and A. 



RESISTANCE TO TUBERCULOSIS. 



Experiments conducted cooperatively by the Bureau of Animal 

 Industry and Dr. Paul A. Lewis, of the Henry Phipps Institute, have 

 been described in another paper (Wright and Lewis, 1921). 



It was found that there was very little relation between age, weight, 

 rate of gain, or sex, and the length of life after inoculation with 

 tuberculosis. These factors combined determined less than 7 per 

 cent of the variation in a very heterogeneous lot. 



